This invention relates to electrography and to a particulate toner composition and a dry electrographic developer composition containing such a toner useful in the development of latent electrostatic charge images.
Electrographic imaging and developing processes, e.g. electrophotographic imaging processes and techniques, have been extensively described in both the patent and other literature, for example, U.S. Pat. Nos. 2,221,776 issued Nov. 19, 1940; 2,277,013 issued Mar. 17, 1942; 2,297,691 issued Oct. 6, 1942; 2,357,809 issued Sept. 12, 1944; 2,551,582 issued May 8, 1951; 2,825,814 issued Mar. 4, 1958; 2,833,648 issued May 6, 1958; 3,220,324 issued Nov. 30, 1965; 3,220,831 issued Nov. 30, 1965; 3,220,833 issued Nov. 30, 1965; and many others. Generally these processes have in common the steps of forming a latent electrostatic charge image on an insulating electrographic element. The electrostatic latent image is then rendered visible by a development step in which the charged surface of the electrographic element is brought into contact with a suitable developer mix. Conventional dry developer mixes include toner or marking particles and may also include a carrier vehicle that can be either a magnetic material such as iron filings, powdered iron or iron oxide, or a triboelectrically chargeable, nonmagnetic substance like glass beads or crystals of inorganic salts such as sodium or potassium fluoride. The toner or marking particles typically contain a resinous material suitably colored or darkened, for contrast purposes, with a colorant like dyestuffs or pigments such as carbon black.
One method for applying a suitable dry developer mix to a charged image-bearing electrographic element is by the well-known magnetic brush process. Such a process generally utilizes an apparatus of the type described, for example, in U.S. Pat. No. 3,003,462 issued Oct. 10, 1961 and customarily comprises a non-magnetic rotatably mounted cylinder having fixed magnetic means mounted inside. The cylinder is arranged to rotate so that part of the surface is immersed in or otherwise contacted with a supply of developer mix. The granular mass comprising the developer mix is magnetically attracted to the surface of the cylinder. As the developer mix comes within the influence of the field generated by the magnetic means within the cylinder, particles thereof arrange themselves in bristle-like formations resembling a brush. The brush formations that are formed by the developer mix tend to conform to the lines of magnetic flux, standing erect in the vicinity of the poles and laying substantially flat when said mix is outside the environment of the magnetic poles. Within one revolution the continually rotating cylinder picks up developer mix from a supply source and returns part or all of this material to this supply. This mode of operation assures that fresh mix is always available to the surface of the charged electrographic element at its point of contact with the brush. In a typical rotational cycle, the roller performs the successive steps of developer mix pickup, brush formation, brush contact with the electrographic element, e.g. a photoconductive element, brush collapse and finally mix release.
In magnetic brush development, as well as in various other types of electrographic development wherein a dry triboelectric mixture of a particulate carrier vehicle and a toner powder are utilized, e.g., cascade development such as described in U.S. Pat. Nos. 2,638,416 and 2,618,552, it is advantageous to modify the triboelectric properties of the toner powder so that a uniform, stable relatively high net electrical charge may be imparted to the toner powder by the particulate carrier vehicle. It is also highly advantageous if the triboelectric properties of the toner powder remain fairly uniform even when the toner powder is used under widely varying relative humidity (RH) conditions.
A variety of methods and material for modifying the triboelectric properties of particulate toner particles have been proposed. For example, Olson, U.S. Pat. Nos. 3,647,696 issued Mar. 7, 1972 describes a uniform polarity resin electrostatic toner containing a mono- or di-functional organic acid nigrosine salt. The nigrosine salt described in U.S. Pat. No. 3,647,696 aids in providing a relatively high uniform net electrical charge to a toner powder containing such a nigrosine salt. However, subsequent testing and development relating to the use of such organic acid nigrosine salts has shown that such materials, when incorporated in a toner composition, contribute to a decrease in the adhesion of the toner particles to a suitable paper receiving sheet. For example, it has been found that when a toner image is transferred from a charge image-bearing electrographic element to a paper receiving sheet and fixed to the receiving sheet, the image formed on the receiving sheets tends to flake off when the sheet is bent or folded.
In addition, Greig U.S. Pat. No. 3,079,272, issued Feb. 26, 1963, describes the use of 4-5%, by weight, of anionic compounds, such as stearic acid, in "melt-form" developer compositions containing particulate toner particles to "improve the triboelectric charge relationship" between the toner particles. However, it has been found that fatty acids, such as stearic acid, when incorporated in dry toner formulations of a magnetic brush developer composition do not enable one to obtain a resultant developer in which a relatively high net positive electrical charge is imparted to the toner particles by the magnetic carrier particles. And, it has also been found that incorporation of fatty acids, such as stearic acid, in toner particles tends to decrease adhesion of such toner particles to suitable plain paper receiving sheets.
Other materials which have been employed as modifying agents for dry toner compositions include various long-chain anionic or cationic materials such as various surfactants. Typical of these surfactant materials are the long chain quaternary ammonium surfactants. The use of such materials is described, for example, in British Pat. No. 1,174,573 published Dec. 17, 1969, at page 2, column 2 through page 3. In addition, Jacknow et. al. U.S. Pat. No. 3,577,345 issued May 4, 1971, describes a solid metal salt of a fatty acid admixed with one of various other described solid additives as a useful modifying combination for a dry toner composition.
Still other materials which have been found useful as charge control agents for electrostatic toner compositions are certain non-surfactant, short-chain, quaternary ammonium salts, such as those described in Research Disclosure, No. 10244, published October 1972 or in Jadwin et al U.S. Pat. No. 3,893,935 issued July 8, 1975, and certain alkoxylated amines, such as those described in Jadwin et al U.S. patent application Ser. No. 470,425, filed May 16, 1974. The aforementioned quaternary ammonium salts and alkoxylated amines have been found capable of providing relatively high, uniform net electrical charge to a toner powder in which these materials are incorporated without any substantial deleterious effect on the adhesion properties of the toner compositions.
Although the above-described non-surfactant short chain quaternary ammonium salts have been found quite useful, it would be desirable to have a charge control agent for a dry electrostatic developer composition which imparts even greater RH stability to the developer composition. In addition, it would be useful to find new charge control agents which, when incorporated in toner, provide other advantageous properties, such as a toner which, when admixed with carrier to form a developer, is capable of improved long-life electrographic print uniformity. Also, it would be desirable to find new charge control agents which exhibit higher decomposition temperatures than certain of the preferred charge control agents of the prior art, such as the above-described quaternary ammonium salts and alkoxylated amines.
In addition to the above-mentioned materials which have been used specifically to modify the triboelectric properties of electrostatic toner particles contained in dry electrographic developer compositions, British Pat. No. 1,169,703, dated Nov. 24, 1966, describes the use of relatively large amounts, i.e., from 2 to about 15 percent by weight, of various ammonium salts, including myristyl dimethyl ammonium ethyl sulphate and cetyl dimethyl ethyl ammonium ethyl sulphate as additives for conducting printing ink particles to increase the electrical conductivity thereof to a level generally less than 10.sup.10 ohm-cm.